1. Field of the Invention
The present invention relates in general to air/fuel ratio control systems of an internal combustion engine, and more particularly to air-fuel ratio control systems of a type which can selectively feed the engine with a mixture of higher (or lean) air/fuel ratio and a mixture of stoichiometric air/fuel ratio.
2. Description of the Prior Art
Hitherto, for improvement in fuel consumption, a so-called "lean-burn engine" has been proposed and put into practical use particularly in the field of wheeled motor vehicles. The lean-burn engine can operate on a mixture which is extremely leaner than a mixture of stoichiometric air/fuel ratio (viz., 14.7/1). For operation of the lean-burn engine, a lean mixture having air/fuel ratio of about 20/1 to 25/1 is usually used. In fact, for the operation, an air/fuel ratio control system is used which, under low rotation and low load condition of the engine, feeds the engine with such a lean mixture for improving fuel consumption, and which, with increase of load of the engine, gradually makes the air/fuel ratio of the mixture richer toward the stoichiometric ratio.
As is known, when the engine operates on a mixture of stoichiometric air/fuel ratio, a three-way catalytic converter, which is installed in the exhaust line of the engine, assuming its activated state can exhibit the maximum conversion efficiency (or purifying efficiency) against CO, HC and NOx contained in the exhaust gas from the engine.
In such air/fuel ratio control systems, there has been proposed a type by Japanese Patent First provisional Publication 62-17341 in which until the temperature of the engine cooling water rises to a predetermined degree, the control is so made that the engine operates on a lean mixture and once the temperature exceeds the predetermined degree, the control is switched over to a control wherein the engine operates on a mixture of stoichiometric air/fuel ratio.
However, even the air/fuel ratio control system proposed by the Publication have failed to exhibit a satisfied performance in emission control. That is, due to its inherent construction, it sometimes occurs that even when the catalytic converter has come into its activated state, the temperature of the engine cooling water does not reach the predetermined degree due to a very cold surrounding or the like. Under this condition, the engine is forced to continue the lean mixture operation, so that the catalytic converter in activated state can not exhibit a satisfied purifying efficiency against the exhaust gas emitted from the engine.
For ease of description, in the following, a control wherein an engine is controlled to operate on a lean mixture will be referred to as "a lean air/fuel ratio control" and a control wherein the engine is controlled to operate on a mixture of stoichiometric air/fuel ratio will be referred to as "a stoichiometric air/fuel ratio control".